The present invention relates to digital communication and more particularly to evaluating channel characteristics.
In digital communication systems, it is often desirable to characterize the channel used for transmitting data from a transmitter to a receiver. The receiver and/or the transmitter may improve communications performance by performing signal processing that requires knowledge of the channel. Particularly in mobile channels, and often in fixed channels, channel characteristics will vary over time and taking advantage of knowledge of channel characteristics requires repeated measurements.
OFDM (Orthogonal Frequency Division Multiplexing) is a desirable modulation scheme for digital communication. In an OFDM transmitter, the inverse Fast Fourier Transform (IFFT) is applied to a burst of N frequency domain symbols to obtain a burst of N time domain symbols. The last v symbols in the time domain are then affixed to the beginning of the time domain burst as a cyclic prefix, increasing the length of the time domain sequence to N+v. The resulting complex digital time-domain sequence is converted to in-phase (I) and quadrature (Q) analog signals that are used to quadrature-modulate an RF carrier. The RF carrier is then radiated from a transmitter antenna.
At the receiver, the signal is captured by another antenna and downconverted into I and Q baseband analog waveforms, that are then converted to digital representations with analog-to-digital converters. The cyclic prefix is removed and the remaining N received time domain symbols are converted to the frequency domain by applying a Fast Fourier Transform (FFT).
Each frequency domain symbol in the burst input to the IFFT can be viewed as corresponding to a different orthogonal narrowband subcarrier. Each subcarrier is low data rate as compared to the burst as a whole. Since the symbol periods are therefore very long, intersymbol interference as would be caused by multipath effects is greatly ameliorated.
Even greater increases in the performance of OFDM systems are possible if the channel characteristics are known. For example, so-called xe2x80x9cvector codingxe2x80x9d techniques may be applied to effectively remove frequency selective effects imposed by the channel. Further highly advantageous spatial processing techniques take advantage of the use of multiple antennas or multiple antenna polarizations at the transmitter and/or receiver. See WO 98/09385, the contents of which are herein incorporated by reference. In the context of such spatial processing it is useful to know the characteristics of a channel which couples a combination of transmitter antenna and receiver antenna.
One way of learning channel characteristics in an OFDM system is to force some of the symbols input to the IFFT to predetermined values. These symbols are then referred to as training symbols. At the receiver end, the channel can be characterized by its effect on the training symbols whose ideal values are known to both transmitter and receiver. Symbols used for training are, however, then unavailable to carry data, reducing system efficiency, particularly in systems where channel characteristics change rapidly enough to require frequent updates or even retraining with every burst.
It has been found that a complete characterization of the frequency response does not require use of a training symbol at each subcarrier frequency. In fact, it is sufficient to use v training symbols within a single burst where v represents the impulse response duration of the overall channel including transmitter and receiver components as well as the propagation medium. To increase data transmission capacity, it would be desirable to decrease the number of training symbols used even further.
The present invention provides a system and method for characterizing a channel used by a digital communication system. In an OFDM system, the number of training symbols used to characterize a channel may be decreased by taking advantage of knowledge of certain channel components, e.g., transmitter and/or receiver digital filters. The number of training symbols used may be less than v, the duration of the impulse response of the overall channel.
According to a first aspect of the present invention, a method for characterizing a channel having known and unknown components includes: receiving a time domain series of symbols via said channel, converting said time domain series of symbols into a series of frequency domain symbols including xcexc training symbols wherein xcexc is less than a duration of an impulse response of said channel but not less than an impulse response of said unknown component of said channel, and processing said training symbols to characterize said unknown components of said channel.